The changes to mileage estimates are being made to appease buyers of vehicles who are unable to match the EPA's estimates. This has been quite a topic of conversation lately, due to the proliferation of hybrid models hitting the marketplace. Numerous web boards have discussions related to actual mileage numbers that users of the vehicles are returning.
It would be good to mention here that the mileage estimates by the EPA are not the same as the Corporate Average Fuel Economy numbers used by lawmakers to enforce mileage regulations on carmakers. Also, while it is possible for drivers to achieve drastically lower numbers than the EPA estimates, it is also possible to meet or exceed them. The numbers are being changed more to reflect drivers' habits than technology problems. The old numbers reported by the EPA were not "wrong", they were real figures which are completely achievable if the vehicle is driven under the same conditions as those the vehicle was tested under. Unfortunately, few drivers drove in a matter even remotely similar to the EPA tests. So, the new numbers are ultimately a good thing for consumers, even though they may make green cars seem a little less so.